In high pressure liquid chromatography, components of a mixture or solution are separated based upon the rates at which they are carried by a liquid mobile phase through a column containing a stationary or bonded phase which is bonded to a support material. The support material is usually porous and usually has its inner and outer surfaces covered with hydroxyl molecules. These hydroxyl molecules are displaced when an organic material bonds to the surface of the porous support material, either physically or chemically, to form the bonded phase. Typically, the porous support material has a surface area which allows about 2.5 molecules of organic material to be placed on each square nanometer of porous support material surface. The order at which various components of a solution elute from the chromatographic column is determined by the nature of the organic material bonded to the surface of the porous support material. The relative concentrations of the components of the solution are determined by calculating the area under peaks printed on a chromatogram.
Currently in the art, the organic material is bonded to the surface of the porous support material by either saturation coverage or starved coverage. With saturation coverage, the organic material is provided in an amount much greater than 2.5 molecules/nm.sup.2 so that the porous support material is swamped with the organic material and as much as possible of the surface area of the porous support material is covered with the organic material. Because unreacted hydroxyls on the surface of the porous support material affect time of elution molecules in the solution and cause broadening of the resultant chromatographic peaks, any unreacted hydroxyls on the surface of the support material are then reacted with a second organic material. Such a process is referred to as "end-capping." Typically, the organic material is added in a concentration sufficient to cap off residual hydroxyl groups which are not bonded with the first organic material. This second, end-capping material plays no part in the order of elution of the components of the solution from the column and is provided solely to prevent unreacted hydroxyls on the surface of the porous support material from extending the elution time or broadening the resultant peaks. Usually, but not always, end-capping agents are C.sub.1 silanes.
With starved coverage, an organic material is reacted with the porous support material in an amount much less than necessary to react with all of the hydroxyls available on the entire surface of the porous support material. The remaining hydroxyls on the surface of the support are end-capped with a second organic material which plays no part in the chromatographic separation. When a starved coverage of the porous support material is used, the elution time of the chromatograph is typically faster than the elution time of a chromatograph employing a saturated coverage because there are fewer molecules on the surface of the support with which the components of the solution can interact.
In U.S. Pat. No. 5,110,784, a novel method is described for providing a porous support material for chromatographic separations. The method includes reacting a stable but rapidly reacting silane with the hydroxyls on the exterior surface of the porous support material, and then reacting a second reactive silane with the hydroxyls on the porous interior surface of the porous support material to provide a dual surface material in which the silicon-containing groups attached to exterior surface of the porous support material are different than the silicon-containing groups on the surface of the porous interior of the material.
However, these prior art materials still experience problems with interference. Interference occurs when the chromatographic peak on a chromatogram for one component of a solution overlaps at least one peak for another component of the solution. Interference results when the chromatographic column is unable to completely resolve, i.e. separate, those two or more components to produce separate peaks. When resolution problems occur, it is common in the art either to increase or decrease the elution time in an attempt to move interfering peaks to a position at which they do not interfere with each other. However, with conventional chromatographic columns and packing materials, increasing or decreasing the elution time does not always remove interfering peaks and, in the case of increasing the elution time, the time required to run the chromatogram greatly increases.
Currently, a need exists in the art for a bonded phase material for a chromatographic separation process which can be used to selectively elute materials in a high pressure liquid chromatographic column so that interference problems can be reduced, if not eliminated entirely.